To operate properly, electrical circuitry often requires a continuous supply of electrical energy. The energy can be provided from a source at one or more nominal voltage levels (e.g., +3.3 volts, v), and current is drawn at these respective voltage levels. The circuitry can operate in a variety of modes each having different associated levels of energy consumption.
A refreshed circuit device includes an operational mode and a refresh mode, with the operational mode relating to its operational interaction with other circuitry and the refresh mode relating to actions taken place to maintain the device in a given state. For example, dynamic random access memory (DRAM) provides an array of storage cells that store electrical charge in order to serve as a memory space for digital data. Data are read from and written to the various cells to carry out a data transfer operation with other circuitry.
Because the storage cells lose the stored charge at a given decay rate (i.e., the storage cells in a DRAM can be characterized as leaky capacitors), a self-refresh operation is carried out in the background in order to maintain the device in the then-existing logical state. During the self-refresh operation, the device reads the state of the array and then rewrites that same state to the various storage cells in the array. In this way, the charge in the various storage cells is continually refreshed at a rate faster than the rate at which charge decays from the cells.
It follows that the state of a refreshed device will generally be lost once the application of electrical power to the device is interrupted. There is therefore a continued need for improvements in the art for providing sufficient and reliable backup power from a battery or other standby source to maintain the then-existing state until normal system operation can be restored. It is to these improvements that the embodiments of the present invention are directed.